But the A.C.A. has not done as much as many had hoped it would to reduce underinsurance. In fact, it may be helping to spread it. And proposed modifications to the law, like those that would introduce a new tier of “copper” plans in addition to bronze, silver, gold and platinum, might make underinsurance worse.

In the most recent update of the Commonwealth Fund survey, conducted in September and October of this year, investigators found that 13 percent of all adults 19-64 spent more than 10 percent of their income on out-of-pocket health care costs. Poor adults were the most likely to spend this amount. More than 30 percent of nonelderly adults earning less than the poverty line spent more than 10 percent of their income on out-of-pocket costs, and 18 percent of those making between 100 percent and 200 percent of the poverty line did so. All of these people were insured.